Appendix: Generalisation of the Concept to Converter-Combiner MMIs with K outputs167. 4.5 Spatial Mode Filters realized with Multi-Mode Interference ...
Diss. ETH No. 12991
Advanced Indium-Phosphide Waveguide Mach-Zehnder Interferometer All-Optical Switches and Wavelength Converters A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH for the degree of Doctor of Natural Sciences by JUERG LEUTHOLD dipl. Physicist ETH Born on July 11th, 1966 Citizen of Nesslau SG, Switzerland
Submitted on the recommendation of Prof. Dr. H. Melchior, examiner Prof. Dr. H. Jäckel, co-examiner Prof. Dr. F.K. Kneubühl, co-examiner
1998
Jürg Leuthold
Advanced Indium-Phosphide Waveguide Mach-Zehnder Interferometer All-Optical Switches and Wavelength Converters
Hartung-Gorre Verlag Konstanz, Germany 1998
“It is the honour of God to conceal things, but the honour of kings to explore things.” (King Solomon, ~990 - 930 B.C.)
To my parents and to my wonderful wife Barbara
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ii
Table of Contents Table of Contents
iii
Abstract
1
1
7
Introduction
1.1 All-Optical Components in Future Terabit-Per-Second Telecommunication Systems 8 1. 2. 3.
Progress towards Terabit-Per-Second Transmissions WDM/OTDM Enabling Technologies Conclusions
1.2 From Early to State-of-the-Art All-Optical Devices 1. 2. 3.
2
Different Nonlinear Materials Techniques to Exploit the Nonlinearities Configurations
13 13 15 18
1.3 Outline
24
1.4 References
26
Theory of MZI Based All-Optical Devices 2.1 Optical Nonlinearities 1. 2. 3. 4. 5. 6. 7. 1. 2. 3.
35 36
The Wave Equation for Nonlinear Optical Media 36 Solutions of the Nonlinear Wave Equation 37 Kramers-Krönig Relations in Nonlinear Optics 39 Nonlinear Effects 40 Time Regimes of Nonlinear Effects 56 Qualitatively Calculated Refractive-Index Changes under Control-Signal Injections59 Concluding Remarks on XPM All-Optical Devices 60
2.2 The Semiconductor Optical Amplifier Equations
3
8 10 12
Introductory Definitions The Propagation of an Amplified Signal in an SOA A More Complete Solution
61 61 65 68
2.3 The MZI-SOA Transfer Functions
71
2.4 References
75
Material Parameters of 1.55 μm bulk InGaAsP Lasers and Amplifiers 81 3.1 Material Gain 1. 2. 3. 4. 5.
82
Introduction Structures and Devices Material-Gain Characterisation Comparison of Experiment with Theory Differential Material Gain dgm/dN and dgm/dT
iii
82 83 84 91 92
3.2 Refractive-Index Changes 1. 2.
95
Measurement of the Refractive-Index Changes Effective Group Refractive Index
3.3 Alpha Factors 1. 2. 3.
103
Introduction Experiments on αN Experiments on αT
103 104 106
3.4 Parametrizations of the Material Parameters 1. 2. 3. 4.
Material-Gain Parametrization Internal-Loss Parametrization Refractive-Index-Change Parametrization Alpha Factor Parametrization
3.5 Appendix 1. 2. 3.
Temperature Correction Current Carrier-Density Relations Output-Power
115 117 119
125
4.1 Summary of Multimode-Interference Theory Self Images and the Term Multimode Interference Classification of MMIs
4.2 Design Guidelines for MMIs 1. 2.
108 112 113 114
121
Modified Multimode-Interference Couplers 1. 2.
108
115
3.6 References 4
95 102
126 126 127
132
MMI Widths and MMI Lengths Trap Waveguides
132 135
4.3 Guided Wave 1.30/1.55 μm Wavelength Division Multiplexers based on Multimode Interference 136 1. 2. 3. 4.
Introduction MMI-WDMs Experiments Future Applications as MMI-WDM Add-Drop Switches
4.4 Multimode Interference Couplers for the Conversion and Combining of Zero and First Order Modes 1. 2. 3. 4. 5. 6.
136 137 143 145
147
Introduction 147 Principles of MMI-Converter-Combiners 148 Power-Splitter for Zero and First Order Modes 157 Experiment 161 Conclusions 166 Appendix: Generalisation of the Concept to Converter-Combiner MMIs with K outputs167
4.5 Spatial Mode Filters realized with Multi-Mode Interference Couplers 170 1.
Introduction
170
iv
2. 3. 4.
Principle of MMI-Filters Experiment Conclusions
171 173 176
4.6 Optical Bandwidths and Design Tolerances of Multimode-Interference Converter-Combiners 1. 2. 3. 4. 5.
Introduction Theory Comparison with Mode Analysis and Experiment Conclusions Appendix
4.7 References 5
177 178 181 185 186
188
All-Optical MZI-Based Devices with Enhanced Extinction Ratios
193
5.1 Extinction-Ratio Limitations in All-Optical Devices
194
5.2 All-Optical Space Switches with Gain and Principally Ideal Extinction-Ratios
196
1. 2. 3. 4. 5. 6.
Introduction Basic MZI-SOA All-Optical Switch Analysis Specific MZI-SOA Implementations Experiments Conclusions
5.3 References 6
177
222
All-Optical Devices with Integrated Data- and Control-Signal Separation Schemes 6.1 All-Optical Mach-Zehnder Interferometer Wavelength Converters and Switches with Integrated Data- and Control-Signal Separation Scheme 1. 2. 3. 4. 5.
196 197 199 204 214 218
Introduction Co- and Counter-Propagating Operation with Versatile All-Optical Devices228 All-Optical Control- And Data-Signal Separation Schemes Static and Dynamic Characterizations Conclusions
225
226 226
230 236 248
6.2 Polarization Independent Optical Phase Conjugation with Pump-Signal Filtering in a Monolithically Integrated Mach-Zehnder Interferometer Semiconductor Optical Amplifier Configuration 249 1. 2. 3.
Introduction Configuration Experiments
249 250 251
6.3 References
256 v
7
Conclusions and Outlook
261
7.1 Conclusions
262
7.2 Outlook App. A App. B App. C App. D App. E App. F
Material Gain Nonlinear Gain Compression InGaAsP Material Parameters Material Parameters Used in the Calculations Useful Formulas List of Symbols and Acronyms
264 Appendix 265 266 273 276 280 281 283
Acknowledgements
289
List of Publications
291
Curriculum Vitae
293
Index
295
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Abstract To meet the future telecommunication-bandwidth requirements, optical-fibre communication systems will be an absolute need. In order to exploit the terahertz bandwidth of optical fibres, generally both of the two current primary techniques for data multiplexing i.e. wavelength-division multiplexing (WDM) and time-division multiplexing (TDM) - are used. While components necessary for implementing a WDM point-to-point link are already on the market, components for TDM links are still in research state. However, the situation tends to change with the occurrence of all-optical components that have the potential for ≥ 100 GHz operation. All-optical components enable highest switching speeds as needed in TDM systems and in addition, they allow all-optical regeneration of pulse streams and the implementation of transparent networks, in which the signals traverse a network consisting of optical fibres and optical nodes without being converted into electrical current, except at the final destination. This thesis introduces novel concepts for all-optical devices based on a Mach-Zehnder interferometer (MZI) configuration with semiconductor optical amplifiers (SOAs) on the MZI arms for high-speed TDM telecommunication. New concepts are applied to provide high extinction ratios and integrated data- and control-signal separation schemes. Beyond, our devices feature high crosstalk, large optical bandwidths and principally polarization insensitivity. The devices are multi-functional, since they may be used for switching, multiplexing, demultiplexing and wavelength conversion. For all-optical processing we focused our attention on the improvement and development of devices based on the cross-phase modulation (XPM) technique. In XPM devices small refractive-index changes induced in a nonlinear medium by a control signal are exploited in a Mach-Zehnder-interferometer configuration to switch a data signal from one output port to the other. Alternative techniques to perform all-optical switching are based on four-wave mixing (FWM) and cross-gain modulation (XGM). Devices based on the XPM technique offer advantages such as high extinction ratio, switching with moderately high control-signal powers and low chirp in comparison with the alternative techniques. The XPM all-optical devices proposed in this thesis use semiconductor optical amplifiers (SOAs) as a nonlinear medium. We apply semiconductor material equations to quantify the different contributions to the nonlinear effects of our devices. As other authors, we find that the dominant nonlinear mechanisms are the bandfilling effect due to gain-saturation, the band-gap shrinkage and the plasma effect. Ultrafast nonlinear effects such as gain compression from spectral hole burning and carrier heat-
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ing or the optical Kerr effect due to the a.c. quadratic Stark effect and two-photon absorption will only contribute to the nonlinearities when the control pulses length is reduced to the picosecond range and the control-signal powers are increased. Relevant material parameters needed for an optimization of the devices are the material gain, the refractive-index change and the alpha factor. We have measured these parameters throughout the gain spectrum and for different carrier densities at various temperatures. Beyond, other important material parameters such as the internal losses, the carrier densities as a function of the applied current and the internal quantum efficiency have been determined. A key issue of any switching device is the extinction or on-off ratio. We show that asymmetries are needed to overcome the extinction-ratio limitations in MZI-SOA all-optical switches. To obtain high extinctions within an interferometric configuration, such as the MZI, both the intensities as well as the phases of the signals have to be properly controlled. However, this is one of the challenges for the all-optical devices, since the strong optical control signal that operates the device, not only induces phase shifts but also modulates the intensities of the data signal to be switched. These extinction-ratio limitations can be overcome by the introduction of asymmetries. The extent of the required asymmetry depends on the SOA material parameters as the alpha factor. For the separation of the data and control signal after signal processing high-performance wavelength multiplexer or filters are needed. We show three new concepts that allow to perform all-optical operation, while at the same time the data and control signal are separated at the output. The first concept is based on a dual-order-mode configuration, in which the data signal propagates as a zero-order mode and the control signal propagates as a first-order mode. Due to the different symmetries of data and control signal, the first-order mode control signal can be easily separated after signal processing and reused. For the introduction and the conversion of the zero-order mode into a first-order mode control signal, we have invented a new coupler type based on multimode-interference (MMI). Excellent performances were found for the new MMI couplers both, for the conversion efficiencies as well as the optical bandwidths. A second concept that allows to separate and to reuse the control signal after signal processing is based on interleaved MZI configurations. Additional MZIs are placed on the arms of an exterior MZI to direct the control signal and the data signal by interference onto separate outputs. A third concept needs 1.55 mm data signals and 1.3 mm control signals to modulate the all-optical device. In contrast to the previous concept, this concept has been introduced by another group. Here we propose compact and simple couplers based on MMI to perform the wavelength multiplexing at the inputs and the outputs. We have realized the first two concepts and have compared them. Depending on the
2
application, each of the concepts has its advantages for specific applications. A recent implementation of our dual-order mode concepts by a global telecommunication company shows the high interest for new solutions in the field of optical highspeed communication.
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Zusammenfassung Um die steigenden Kapazitätsanforderungen in der Telekommunikation bewältigen zu können, werden immer mehr Glasfaserkommunikationssysteme eingesetzt, denn die Signalbandbreite von optischen Glasfasern reicht in den Terahertzbereich. Damit man diese Bandbreite auch ausschöpfen kann, wird sowohl die Technik des Wellenlängenmultiplexens (WDM) als auch die Technik des Zeitmultiplexens (TDM) angewendet. Währenddem Komponenten, welche für das WDM benötigt werden, bereits kommerziell erhältlich sind, befinden sich die Komponenten, welche für das TDM gebraucht werden, noch immer im Forschungsstadium. Die Situation könnte sich jedoch mit dem Aufkommen von optisch-optischen Bauteilen verändern. Diese können mit Schaltgeschwindigkeiten von weit über 100 GHz betrieben werden. Ausserdem erlauben sie die optisch-optische Signalaufbereitung und die Installation von transparenten Netzwerken. In transparenten Netzwerken werden die Datensignale ausschliesslich via Glasfasern und rein optische Schalterzentralen übertragen, ohne dass sie unterwegs in elektrische Signale umgewandelt werden. In der vorliegenden Dissertation werden neue Konzepte von Schaltern, welche auf optisch-optischen Mach-Zehnder-Interferometern (MZI) basieren und über optische Halbleiterlaserverstärker (SOA) verfügen, eingeführt und getestet. Neue Konzepte werden benötigt, um bessere Schaltverhältnisse zu erzielen und um die zum Betrieb verwendeten Kontrollsignale bereits innerhalb des Schalters von den Datensignalen zu trennen. Darüber hinaus verfügen die neuen Schalter über eine grosse optische Bandbreite und erlauben einen polarisationsunabhängigen Betrieb. Ausserdem sind die Bauteile auf vielfältige Weise einsetzbar, da sie sowohl zum Schalten, Multiplexen, Demultiplexen wie auch für die Wellenlängenkonversion geeignet sind. Zum Betrieb der optisch-optischen Schalter machen wir Gebrauch von der sogenannten Kreuzphasenmodulations(XPM)-Schalttechnik und konzentrieren unsere Aufmerksamkeit vor allem auf die Entwicklung und Verbesserung dieser Technik. XPM-betriebene Schalter bestehen zur Hauptsache aus einer MZI-Konfiguration mit nichtlinearen Materialien auf den MZI-Armen. Wenn nun ein Kontrollsignal in einem nichtlinearen Bereich auf einem MZI-Arm eine Brechungsindexänderung erzeugt, können dank der MZI-Konfiguration kleinste Brechungsindexänderungen zum Schalten ausgenutzt werden. Alternative Techniken zum Betrieb von optischoptischen Schaltern basieren auf der Technik des Vierwellenmischens (FWM) oder auf der Technik der Kreuzverstärkungsmodulation (XGM). Allerdings bieten Bauteile, die auf der XPM-Technik basieren, gegenüber diesen alternativen Techniken einige wesentliche Vorteile. Zum Beispiel erreicht man mit XPM-betriebenen Bauteilen bessere Schaltverhältnisse. Ausserdem erlauben sie das Schalten mit kleineren Kontrollsignalstärken und kleinerer Frequenzverschiebung.
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Die auf XPM basierenden optisch-optischen Schalter, wie sie in dieser Dissertation eingesetzt werden, verwenden optische Halbleiterverstärker als nichtlineares Material. Wir gebrauchen die optischen Halbleiterverstärkergleichungen, um in unseren Bauteilen die verschiedenen nichtlinearen Anteile am Schaltvorgang zu quantifizieren. Wie bereits andere Autoren vor uns haben wir gefunden, dass die dominanten nichtlinearen Effekte von der Ladungsträgersättigung, der Verschiebung der Energiebandkante des Halbleiters und vom Plasmaeffekt herrühren. Ultraschnelle nichtlineare Effekte wie “Spektrales-Loch-Brennen” (SHB), Ladungsträgererwärmung (CH) oder der optische Kerr-Effekt aufgrund des quadratischen Stark-Effektes und der Zwei-Photonen-Absorption werden erst für stärkere Kontrollpulse unter ~1ps wirksam. Wichtige SOA-Materialparameter, welche zur Optimierung der Bauteile benötigt werden, sind die Materialverstärkung, die Brechungsindexänderungen und der sogenannte Alphafaktor. Wir haben diese Parameter in unseren SOA gemessen. Die Messungen wurden über den ganzen verstärkungswirksamen spektralen Bereich, bei verschiedenen Stromstärken und bei verschiedenen Temperaturen durchgeführt. Darüber hinaus wurden auch andere wichtige Parameter bestimmt wie zum Beispiel die internen Verluste, die Abhängigkeit der Ladungsträgerdichte vom angelegten Strom sowie die internen Quanteneffekte. Die erzielbaren An-/Abschaltverhältnisse stellen generell ein wichtiges Kriterium zur Beurteilung der Schaltqualität dar. Wir zeigen, dass man die An-/Abschaltverhältnisse der auf MZI-SOA basierenden optisch-optischen Schalter durch das Einführen von Asymmetrien erhöhen kann. Um mit einer MZI-SOA-Konfiguration gute Schaltverhältnisse zu erzielen, muss man sowohl die Phasenverhältnisse als auch die Intensitäten der Lichtsignale auf den beiden MZI-Armen gegeneinander abstimmen. Aber genau das ist die Schwierigkeit in optisch-optischen Schaltern, denn das starke Kontrollsignal, das zum Schalten benötigt wird, moduliert nicht nur die Phase, sondern auch die Intensität des zu schaltenden Datensignals. Durch das Einführen von Asymmetrien können derartige Begrenzungen der Schalteigenschaften überwunden werden. Die benötigte Asymmetrie hängt von Materialeigenschaften wie dem Alphafaktor ab. Für das Herausfiltern des Kontrollsignals aus dem Datensignalpfad werden effiziente Wellenlängenfilter oder Wellenlängenmultiplexer benötigt. Wir zeigen drei neue Konzepte, die es erlauben, optisch-optische Schalter zu betreiben und gleichzeitig die Daten- und Kontrollsignale am Ausgang des Bauteiles zu trennen. Das erste Konzept basiert auf einer “Zwei-Ordnungsmoden-Konfiguration”. In dieser Konfiguration werden die Kontrollsignale in Moden höher Ordnung umgewandelt, während man für die Datensignale nach wie vor Moden nullter Ordnung verwendet. Da die Symmetrie der beiden Moden total verschieden ist, kann diese Eigenschaft ausgenützt werden, um die Moden nach der Signalverarbeitung voneinan5
der zu trennen. Um die Kontrollsignale in höhere erste Ordungsmoden umzuwandeln und sie dann in den Signalpfad der Datensignale einzukoppeln, haben wir neuartige Koppler erfunden. Diese basieren auf dem Multimode-Interferenz(MMI)-Prinzip. Für diese Koppler haben wir hohe Umwandlungs- und Kopplungseffizienzen, aber auch grosse optische Bandbreiten gefunden. Ein weiteres Konzept, um Kontroll- und Datensignale zu trennen, basiert auf mehreren ineinander verschachtelten MZI-Konfigurationen. Dabei wird je eine zusätzliche MZI-Konfiguration auf den MZI-Armen einer äusseren MZI-Konfiguration plaziert. Diese werden gebraucht, um das Kontroll- und das Datensignal interferometrisch auf verschiedene Ausgänge zu führen. Ein drittes Konzept verwendet 1.55 mm Datensignale und 1.3 mm Kontrollsignale zur Modulation des Schalters. Im Gegensatz zu den vorangegangen Konzepten wurde dieses Prinzip bereits von einer andern Gruppe demonstriert. Wir zeigen im Rahmen dieser Dissertation, dass kompakte, einfache MMI-Koppler existieren, die das Einkoppeln und Multiplexen am Ein- und am Ausgang des Bauteils ermöglichen. Die ersten beiden optisch-optischen Schalterkonzepte wurden in InGaAsP/InP realisiert, getestet und miteinander verglichen. Jedes der beiden Konzepte hat seine Vorund seine Nachteile für spezifische Anwendungen. Eine erst kürzlich erfolgte erfolgreiche Realisierung eines unserer Konzepte durch einen globalen Telekommunikationsanbieter zeigt das grosse Interesse an neuen Lösungen im Gebiet der optischen Hochgeschwindigkeitstelekommunikation.
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1 Introduction A short overview on the latest developments enabling terabit-per-second telecommunication is given. One of the enabling technologies for future terabit-per-second networks will be all-optical devices. We show some of the potential applications for this new class of devices. Special emphasis is put on the all-optical devices. We briefly review different nonlinear materials, that allow us to build all-optical components and compare their performance. The largest nonlinearities are found in semiconductor optical amplifiers. Various techniques have been suggested to exploit the nonlinearities for all-optical operation. A most promising technique is cross-phase modulation. Important concepts based on these techniques are studied in more detail at the end of this chapter.
7
1 Introduction
1.1 All-Optical Components in Future Terabit-PerSecond Telecommunication Systems High-capacity telecommunication networks will play an important role in the forthcoming information society of the 21st century. The information communications as well as health care, social welfare, energy systems, traffic etc. will depend on reliable, high-speed transmission networks. The predicted requirements for the first decade of the next century are transmission rates of 5 TBit/s for the backbone networks and 100 MBit/s for homes as stated in a recent visionary report by Hirahara et al. [1.1]. And indeed, a constant capacity increase of 35-60% on a yearly base is predicted [1.2]. Capacity requirements are boosted by the increasing number of users, a move towards an information-oriented lifestyle and the increased bandwidth demand of new services. Especially the real-time communication services pose high requirements on a network, since they need the same high-transmission capacity at all times. If the transmission of a digitally transmitted voice signal requires 64kBit/s , new compressed real-time media demand 128 kBit/s for hi-fi music, 1.5 MBit/s for cumulative video images, 6 MBit/s for commercial TV images and even 30 MBit/s for high-definition TV images [1.1].
1. Progress towards Terabit-Per-Second Transmissions Necessary technological innovations towards terabit-per-second transmissions include the move from copper to silicon fibres and the transition from space to wavelength multiplexing towards time-division multiplexing (Fig. 1.1). While the former revolution initiated the step from the electrical to the optical domain, the latter might initiate progress from optoelectronic towards all-optical transmission equipment. The replacement of copper by optical fibres closer and closer to the residential customers creates the conditions required for the coming terabit-per-second age. Only optical fibres offer an unprecedented capacity bandwidth of as much as 25 THz in combination with minimum attenuation losses as low as 0.2 dB/km for wavelengths around 1.55 μm. Alternative transmission means such as the copper coaxial cable and wireless distribution have much smaller transmission capacities. Copper coax cables have losses of ~2.5 dB/km at modulation frequencies of 1 MHz and more than 50 dB/km at 1 GHz [1.3]. Even with the introduction of new copper cables that provide reliable 1 GBit/s transmission as recently announced by Lucent Technologies [1.4], the transmission capacity remains a factor of 25’000 smaller in comparison with optical fibres. Wireless video-broadcast services suitable for long-distance distributions operate at ‘moderate‘ 2.5 GHz. Faster modulated millimetre-wave systems operating between 28 GHz and 40 GHz have a restricted cell size of 1-5 km and low earth orbit satellites at an altitude of 1500 km offer throughputs of 4 – 5 GBit/s .
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1.1 All-Optical Components in Future Terabit-Per-Second Telecommunication Systems
The second necessary innovation involves the transition from optical space-division multiplexing (OSDM) to wavelength-division multiplexing (WDM) towards optical time-division multiplexing (OTDM). At the beginning of the optical revolution, OSDM was exploited in order to make use of the space domain by using different fibres each carrying a single wavelength. Meanwhile, WDM techniques have entered the market to exploit the frequency domain by multiplexing different wavelengths over a single fibre. Although the technique is relatively new, all major US long-distance carriers are utilising WDM equipment in their networks since 1997 [1.2]. It is foreseeable, that OTDM will be exploited next. OTDM systems in combination with WDM systems will allow to exploit the available optical bandwidths of silicon fibres and thus satisfy the demand for increased capacities. In addition, OTDM systems will allow a user to burst very high-speed (>100 GBit/s) data packets onto the network whenever the network traffic allows it. This is in contrast to a WDM system where each user can access only a small portion of the total bandwidth of the network at any given time, since in a WDM system the fibre bandwidth is divided into a large Innovations
Necessary Technological Developments
Electronic Transmission
Optical Transmission OSDM
Optical Transmitter Optical Amplifier Optical Receiver
WDM
Multiwavelength Sources Multiwavelength Amplifiers Wavelength Mulitplexers (Phased Array) Optical Filters
OTDM
Short Optical Pulse Generation Dispersion Compensation All-Optical Multiplexing/Demultiplexing High Speed Clock Extraction
Fig. 1.1 Progress towards terabit-per-second networks goes along with innovations, such as a move from electrical to optical transmission and a transition from space (OSDM) to wavelength (WDM) to optical time-division multiplexing (OTDM). Each innovation requires some necessary technological developments. After the foreseeable transition from WDM to WDM/ OTDM transmission all-optical multiplexing will become a key technology enabling high-capacity transmission. 9
1 Introduction
number of lower-rate channels [1.5]. The feasibility of terabit-per-second transmission was for the first time demonstrated in spring 1996 when simultaneously three terabit-per-second transmission experiments were reported, see Ref. [1.6-1.8] or [1.9] for a review. WDM/OTDM techniques were applied. In the autumn 1996 a group of NEC further expanded the demonstrated transmission capacity to 2.6 TBit/s over 120 km single-mode fibre using 132 channels, each modulated at 20 GBit/s [1.10]. All these experiments succeeded in high-speed point-to-point WDM/OTDM transmission. In practice, however, networks will be required. For this purpose highspeed switches, multiplexers, demultiplexers and wavelength converters will be needed. All-optical components have the potential to work beyond the limits encountered by electronics and will most probably be a prerequisite in future highspeed WDM/OTDM systems.
2. WDM/OTDM Enabling Technologies Generally, innovations in high-tech markets are based on one or more technological breakthroughs. In Fig. 1.1 we have listed representative necessary developments enabling new multiplexing technologies. Enabling technologies for WDM systems were the development of the erbiumdoped fibre amplifier (EDFA) and the invention of the phased array providing a practicable wavelength multiplexer. The exploitation of the low-loss regions of the silica fibres relies on the availability of high-performance amplifiers in the respective wavelength regions. Nowadays mainly the lowest-loss 1.55 μm range of the silica fibre is exploited. This has been made possible by the invention of the erbium-doped fibre amplifier (EDFA) in 1987, which made the 1.53 to 1.56 μm region accessible [1.11]. EDFAs routinely exceed amplification of 40 dB with a noise figures of 4-5 dB. The 1.53 to 1.56 μm window corresponds to a bandwidth of 3-4 THz, which was nearly completely exploited by the previous mentioned 2.6 THz experiment [1.10]. Lately, new gain-shifted EDFAs make the 1.57 to 1.60 μm region accessible [1.12]. These amplifiers provide a net gain of 38 dB with noise figures below 4.5 dB [1.13]. More recently, the well-known SOAs seem to be rediscovered especially for exploiting the 1.31 μm transmission window [1.14]. Optical fibre-to-fibre gains of typically 30 dB and intrinsic noise figures of 6-8 dB are achieved [1.15-1.17]. Overall noise figures at the gain peak are around 11 dB [1.15,1.16]. In principle, SOAs can be built for use throughout the 1.00 to 1.60 μm range. Raman amplifiers also address the 1.30 μm window. They allow gain levels of 40 dB with noise figures of 4.3 dB [1.18, 1.19]. Other amplifiers working around 1.30 μm encompass fibre amplifiers with active rare-earth ions like the Praseodymium-Doped Fluoride Fibre Amplifier (PDFFA) [1.20].
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1.1 All-Optical Components in Future Terabit-Per-Second Telecommunication Systems
Table 1.1: High-Performance Amplifiers Type
Wavelength Range [μm]
Fiber-Fiber Gain [dB]
Intr. Noise Figure [dB]
EDFA
1.53 - 1.56
40
4-5
Gain-Shifted EDFA
1.57 - 1.60
38
4.5
SOA
1.55 1.30 and others
29 30
6-8 6-8
Raman Amplifier
1.30 - 1.32 and others
40
4.3
PDFFA
1.28 - 1.32
30
Saturation Point [dBm]
7.4 10
Key technologies needed for the current transition from WDM to WDM/OTDM systems are short-pulse generation, dispersion-slope compensation, all-optical multiplexing and clock extraction from high-speed OTDMs [1.3,1.21]. Other developments such as wavelength converters are desirable but not necessary. Several methods are studied to generate short pulses that are synchronized with a master clock. Examples are the mode-locked laser diode [1.22], external modulation of continuous wave (cw) light by an electro-absorption modulator [1.23] and harmonic mode-locking of Er-doped fibre ring lasers [1.24]. Dispersion-slope compensation techniques become essential at highest bitrates, since dispersion tolerance together with fibre-nonlinearity effects decreases as 1,4 ∼ ( BitRate ) increases [1.25]. Table 1.2 gives an idea of the tolerances at different bit rates. Table 1.2: Simulated Dispersion Tolerance (5% Eye Merging Closure), cf. [1.25] Bit Rate
Dispersion Tolerance
10 GBit/s
< 450 ps
20 GBit/s
< 200 ps
40 GBit/s
< 60 ps
In high-speed OTDM networks, all-optical devices are prerequisite for multiplexing, demultiplexing and various signal processing functions [1.26]. In the operation range between 10 and future 100 GBit/s all-optical devices have already proved that they
11
1 Introduction
can overcome the bandwidth bottlenecks of electronic circuitry. They exploit ultrafast nonlinear effects to control light by light. An overview of the different techniques will be given in the next section. The main advantages of all-optical processing in comparison with electronic processing are that it • enables ultrafast operation: Currently all-optical devices have performed switching windows as short as 200 fs at 10.5 GHz [1.27] and 100:10 GBit/s multiplexing [1.28, 1.29]. • allows wavelength conversion [1.32] besides switching [1.30] and multiplexing [1.31], if adequate configurations and techniques are used. • permits multiplexing in combination with clock recovery [1.33, 1.34]. • enables an implementation of all-optical 3R regeneration techniques [1.35-1.38]. The expression “3R” refers to amplification, re-shaping and re-timing. • permits operation of transparent networks. I.e., networks in which a signal is not converted into an electronic signal during transmission. In contrast, present nontransparent networks stop an optical signal in its travel along a fibre by converging it into an electronic signal. They then perform switching and use the electric signal to drive a semiconductor laser, which reproduces the original signal. If there were no physical limitations, such a network could be transparent to channel wavelength and digital-modulation format as well as being independent of bit rates [1.39]. • allows the combination of high-speed multiplexing with high-speed optical logic [1.40-1.42]. • permits signal processing with storage functions [1.43].
3. Conclusions Presumably, the technological progress in the next few years will mainly concentrate on two fields. On one hand the usable bandwidth of optical fibres will be further expanded. On the other hand OTDM techniques will mature. OTDM systems are already in an experimental stage. The necessary technological innovations are under the way. All-optical devices as proposed in this thesis may be the key enabling technologies for practical applications of multiplexing. Successful field trials with all-optical components have already been performed [1.44]. The extension of the usable optical fibre-bandwidth will open new opportunities to apply OTDM techniques.
12
1.2 From Early to State-of-the-Art All-Optical Devices
1.2 From Early to State-of-the-Art All-Optical Devices The intention of this section is to highlight the technological landmarks leading to practical devices exploiting the optical nonlinearities for ultra-fast high-speed all-optical switching. It is useful to first classify the different concepts and to treat each of the classes separately. The classification is depicted in Fig. 1.2. We can distinguish the different concepts with respect to the nonlinear materials (Kerr, SOA, etc.), to the techniques used to exploit the nonlinearity (XPM, XGM, FWM, DFG) and, last but not least, to the configurations (fibre loops, MZI, MI, single SOA). All-Optical
Fibres with Kerr Effect
XPM
Fibre Loops
XPM
MZI
MZI
χ(2) Materials
SOA
MI
XGM
FWM
DFG
Single SOA
Single SOA
Single Device
New Wave Generation
Gating Methods
Fig. 1.2 Classification of all-optical concepts. Devices based on three different materials are employed, primarily. There are Kerr materials, SOA materials, and materials with a strong second-order nonlinearity. The techniques to exploit the nonlinearities are based on cross-phase modulation (XPM), cross-gain modulation (XGM), four-wave mixing (FWM) or difference-frequency generation (DFG). They are applied in different configurations.
1. Different Nonlinear Materials Mainly three different materials provide optical nonlinearities sufficient for all-optical operation: Kerr based optical fibres, SOAs and materials with a strong second(2) order nonlinear susceptibility χ . Nonlinear effects in optical fibres are due either to changes in the refractive index by intense light or to Brillouin and Raman scattering. The power dependence of the re-
13
1 Introduction
fractive index is of particular interest, since it can be exploited for switching in an interferometric configuration. Phenomena, where the refractive index changes Δn' are caused by an electric field, i.e. a light intensity I , are generally referred as Kerr effects, described by: Δn' = I ⋅ n' 2
(1.1)
with the second-order nonlinear refractive index n' 2 . A typical value of n' 2 in a nondispersion-shifted single-mode fibre for 1.55 μm radiation is given in Table 1.3. Kerr-based nonlinearities in fibres are extremely fast. However, it was recognized that the use of active nonlinear elements like SOAs provides much larger nonlinearities [1.45]. SOAs exhibit at least three different nonlinear regimes. The strongest nonlinearities are due to carrier-related refractive-index changes from gain saturation. In order to compare the Kerr effect in optical fibres with the refractive-index changes in SOAs we represent the SOA nonlinearity in terms of n' 2 in Table 1.3. It shows that the carrier-related refractive-index changes in SOAs are by orders of magnitude larger than in fibres. We emphasize that this value may be used only for comparison. The real value depends on the wavelength, the material composition etc. Other nonlinearities in SOAs are either due to gain compression (Spectral Hole Burning, Carrier Heating, ...) or due to SOA related Kerr effects. These effects are usually weaker. Recently, AlGaAs waveguide devices were produced with alternating crystal orientation such that Type-II phase matching for difference-frequency generation (DFG) was obtained [1.46]. The published related experiments require strong pump pulse powers, e.g. 65 mW at 0.78 μm, and exhibit conversion efficiencies of -16 dB. Table 1.3: Comparison between Fiber and SOA Nonlinearities at 1.55 μm n′ 2 [cm2/W] Opt. Kerr Effect of a Standard Fibre SOA (at bandgap)
n′ 2 = 2,2 – 3,4 ⋅ 10 Ref.: [1.47-1.48]
Relaxation Times [ps] – 16
–9
Carrier Effect
n′ 2 = 1,0 ⋅ 10 Ref.: [Chapter 6.1]
Gain Compression
n′ 2 = 6,0 ⋅ 10 Ref.: [Chapter 2.1]
Kerr Effect
n′ 2 = 1,0 ⋅ 10 Ref.: [Chapter 2.1]
– 10
– 12
14
